The invention is directed to a highly wear-resistant iron-nickel-copper-molybdenum sintered alloy which also contains phosphorus.
The method of making highly wear-resistant machinery components from chilled cast iron is well known. Chilled cast iron is an iron-carbon alloy, in which the carbon and silicon contents, aside from the other elements of manganese, phosphorus and sulfur, as well as the nickel and chromium contents are adjusted so that the cast piece solidifies either completely white due to cooling in foundry sand or with only a surface layer white, due to the action of quenching plates. The carbon is thus not precipitated as graphite. The structure consists then of ledeburite with cementite or disintegrated austenite. Chilled cast iron belongs to the best known, most highly wear-resistant alloys. The wear resistance is generally attained due to the cementite and less frequently due to the martensite. The latter can be obtained by appropriately alloying or by quenching. Chilled cast iron practically cannot be deformed.
Although this material has proven to be very successful for highly wear-resistant machine components, it does have the disadvantage that the production of chilled cast iron parts cannot yet be automated. The production of such parts is thus very expensive, especially if the parts are commodity articles, which must be produced in large numbers.
Powder metallurgy has proven to be successful for the production of commodity articles with designated and specified properties. For producing high-strength workpieces by this technique, an iron-molybdenumnickel sintered alloy with addition of phosphorus was developed (German Pat. No. 2,613,255, Austrian Pat. No. 361,959). The objects, produced from this alloy, have a tensile strength of 600 N/mm.sup.2 and higher. These parts are produced using the simple sintering technique and, moreover, without an additional heat treatment. Admittedly, workpieces produced from these alloys attain the desired tensile strength; however, they do not attain the wear resistance of chilled cast iron parts.
For cams of cam shafts, for which a high wear resistance is required, a sintered alloy was developed, which contains chromium, molybdenum, copper, phosphorus and carbon (British Offenlegungsschrift 2,073,247, Hoganaes PM seminar report of March 1985). Comparison tests were carried out, in which chilled cast iron cam shafts and cam shafts with sintered cams of the aforementioned material were subjected to the same testing conditions. The wear values, so obtained, are of comparable orders of magnitude. However, the sintered alloy used here cannot be produced simply by mixing the corresponding elementary metal powders, but must be used as a prealloyed powder, because of the high oxygen affinity of chromium. If elementary chromium were to be mixed in as a powder, an oxide casing would form around the particles before the actual sintering process, because the inert gases used in industry generally are contaminated with oxygen. The oxide casing prevents the diffusion-controlled alloying process.
For the production of prealloyed powders, an alloy of the desired composition is fused and, according to the usual method, atomized to a powder. By carrying out this process under a inert gas of high purity, it is made certain that the element chromium, which has a high affinity for oxygen, dissolves in the alloy. The powder, so obtained, is mixed with elementary carbon (graphite), pressed and sintered. During the sintering process, the chromium forms carbides, which appreciably improve the wear resistance. The interaction of phosphorus and carbon causes a liquid phase to be formed and thus increases the sintering activity.
Parts produced from this prealloyed iron powder have a high shrinkage. The particles of the powder are very hard and can therefore be compressed only with difficulty. Shrinkage in the longitudinal direction is of the order of 5%. In the production of cams for cam shafts, this shrinkage is not entirely undesirable, because it causes the cam to be seated firmly on the shaft. On the other hand, because of the high shrinkage, close tolerances can be adhered to only at great expense, if at all. The production of a prealloyed powder is a sophisticated and therefore expensive process.